This invention relates to single phase or polyphase electrical machines, and more particularly, to single layer interspersed concentric stator winding patterns for turbine generators and construction assemblies of SLIC windings.
The fundamental elements common to a dynamo-electric machine or generator are a stator and a rotor. The stator includes a plurality of slots which are angularly and equally displaced along it""s inner periphery. The rotor has one or more pairs of rotor magnetic poles which are created by permanent magnets or by a current carrying winding, or both. A current carrying winding is placed in the stator slots so as to create magnetic pole pairs on the stator which equal the number of rotor pole pairs. This winding is referred to as a stator winding. The stator for an ac machine can be wound for one or more phases (typically three). The peripheral span of one rotor pole or stator pole is called the xe2x80x9cpole pitchxe2x80x9d.
Stator windings are generally differentiated as single layer windings or two-layer windings. Lap windings and concentric windings are generally known with respect to coil arrangements in three-phase armature windings. Concentric windings have been used on single phase fractional horsepower motors. Concentric windings, however, are rarely used on larger horsepower polyphase machines (ac). Concentric windings are typically used for stator windings on single phase fractional horsepower motors (ac) and for field (rotor) windings for turbine generators (dc).
In a lap winding, coils having substantially similar configurations and peripheral spans are placed one upon the other (typically in a two-layer arrangement) in a sequence, and laid in the slots of a stator core. Thus, electrical characteristics of each phase are balanced since the coils have similar configurations and winding resistances for each phase.
In concentric windings, a plurality of coils having different peripheral spans are laid in the stator core slots such that the coils are distributed to lie concentrically about a pole center. Nested coils are generally located in adjacent slots in prior approaches.
A traditional xe2x80x9cconcentricxe2x80x9d or xe2x80x9cchainxe2x80x9d wound stator winding is one in which a phase winding includes a finite number of xe2x80x9cnestedxe2x80x9d coils which are centered on the axis of the phase winding. The coils each have one or more turns of electrically conductive material (usually copper) which are interconnected in series with each other to form what is termed a xe2x80x9cphase groupxe2x80x9d. The interconnection of the coils in the xe2x80x9cphase groupxe2x80x9d is made so that current will always flow in the same xe2x80x9csensexe2x80x9d in each coil (either always in clockwise direction or always in counter-clockwise direction when viewed radially from the machine axial centerline).
To reduce harmonic content in the stator winding terminal voltage waveform and in the resultant stator winding magnetomotive force (MMF) waveform, usually a second xe2x80x9cphase groupxe2x80x9d of coils is located one pole pitch away from the first xe2x80x9cphase groupxe2x80x9d and is connected with the opposite xe2x80x9cpolarityxe2x80x9d or opposite xe2x80x9csensexe2x80x9d from the first xe2x80x9cphase groupxe2x80x9d of coils. If the machine has more than two poles, then an additional phase group is added for each pole with connections made to reverse the polarity or current sense at each successive pole from that of the previous pole. The phase groups for the phase winding can then be carefully interconnected either in series, parallel, or series-parallel configurations via external conductors known as xe2x80x9cphase connectionsxe2x80x9d. In addition, both conductors at the ends of the now interconnected phase winding are brought outside of the machine by additional phase connections so as to interconnect with an external power system.
In a traditional concentric winding, the xe2x80x9cnestedxe2x80x9d coils in a xe2x80x9cphase groupxe2x80x9d include an innermost short span coil which spans a smaller fraction of a pole pitch which is contained within longer span coils, each of the coils are progressively larger in span by two stator slots. The outermost coil in the nest usually spans a full pole pitch. The conductors of the phase winding of a traditional concentric winding are always located in xe2x80x9cadjacentxe2x80x9d stator slots. Most of the concentric windings in which alternating current (ac) flows have continuously wound phase windings. In other words, the phase winding coils are wound from continuous strands of conductors and no joints are made in the conductors except at the ends of the phase groups.
Polyphase concentric windings have additional phase windings identical to the first phase winding, but displaced on the periphery of the stator from the first phase winding by an angular span which is dependent on the number of phases and number of poles. The most common type of polyphase machine is the three-phase machine. In a three-phase machine, the second phase winding is physically displaced from the first phase winding by (120)(2)/Np degrees, and the third phase winding is physically displaced from the first phase winding by (240)(2)/Np degrees where xe2x80x9cNpxe2x80x9d represents the number of poles on the machine.
In single phase or polyphase rotating machines, one of the design considerations is the reduction in higher order harmonic magnitudes in the resultant stator winding MMF waveform. This is one of the desirable considerations of the present invention. The following description provides some background information on stator winding MMF for concentric windings. For example, consider a 2-pole, three-phase machine for analyzing the case of a dc (direct) current flowing through a single coil of a phase group. Further, assume that the coil spans a full pole pitch of the machine. If the coil MMF, represented an (Y-axis), is plotted as a function of the peripheral angular span of the inner surface of the stator core, represented an (X-axis), the MMF waveform would appear as a rectangular waveform having equal positive and negative amplitudes about a horizontal X-axis. In addition, the angular span of the positive half of the waveform would be identical to that of the negative half of the waveform. Each half of the waveform is determined to span one pole pitch.
Considering the case of another coil in the same phase group, except that this coil spans a small fraction of a pole pitch. Further, assuming that the dc current in this coil flows in the same xe2x80x9csensexe2x80x9d as in the full pole pitch coil. The MMF for this coil would also be rectangular in shape, but is different from that of the full pole pitch coil. The fractional pitch coil has a positive amplitude which is larger than the negative amplitude about the horizontal X-axis. In addition, the angular span of the positive portion of the waveform would only be within the confines of the coil span. The negative portion of the waveform would span the remainder of the stator periphery outside of the fractional pitch coil span.
Similar logic may be applied to other fractional pitch coils within the phase group except that the positive and negative amplitudes would be different and the angular spans of the positive and negative portions of the waveforms would be different, due to the different span of each of these coils.
The second phase group of coils on the adjacent pole would have similar waveforms except that they would be displaced from those of the first phase group by one pole pitch. In addition, the waveforms of the second phase group would be xe2x80x9cinvertedxe2x80x9d (mirror image about the horizontal X-axis) from those of the first phase group because the second phase group is connected with opposite polarity or opposite current xe2x80x9csensexe2x80x9d.
The combined or resultant MMF waveform of all the nested coils in the phase winding may be determined by summing the MMF contribution of each coil at every point on the inner periphery of the stator core. A stair stepped pattern above and below the horizontal X-axis would result for the case of dc current in the phase winding. This stair-stepped xe2x80x9cspacexe2x80x9d pattern resembles a sinusoidal waveform. A Fourier Series Analysis may be made of the resultant phase winding MMF pattern in order to determine the magnitudes of the xe2x80x9cfundamentalxe2x80x9d component and of the various higher order xe2x80x9cspacexe2x80x9d harmonic components.
If alternating current (ac) is applied to the phase winding, then the amplitude of the MMF waveform would continuously pulsate with various amplitudes between the waveform""s maximum positive and negative amplitudes. The ac current introduces xe2x80x9ctimexe2x80x9d dependence into the phase winding MMF waveform. The other two phase windings of a three-phase winding, and their associated MMF waveforms would be displaced in space by 120 degrees and 240 degrees, respectively from the first phase winding. In addition, phase winding currents of the second and third phases would be of identical magnitude but out of phase in time by 120 degrees and 240 degrees, respectively from the current in the first phase winding.
The three individual phase winding MMF waves would combine into an overall resultant MMF wave for the stator. With balanced three-phase currents, the xe2x80x9cfundamentalxe2x80x9d components of the phase winding MMF""s would combine to create a resultant constant amplitude traveling MMF wave which travels at synchronous speed in the direction of rotor rotation. For a synchronous machine, the speed of the rotor and that of the resultant fundamental MMF field are identical, so that there is no relative motion between the two. However, the various other combinations of Fourier series space and time harmonics would result in additional harmonic rotating magnetic fields which are of different amplitudes and rotating in different directions and speeds relative to the rotor. These harmonic MMF waves will induce voltages and resultant currents and losses (typically referred to as xe2x80x9cshort-circuit pole face lossesxe2x80x9d) on the surface of the synchronous machine rotor. These harmonic rotor surface currents may result in excessive heating of the rotor surface and rotor windings, and thus may potentially damage overlapping contact joints between current carrying components.
Therefore, it is desirable to reduce the higher order harmonic content or reduce the magnitude of the higher order harmonic components of the stator winding MMF waveform to enhance the performance of rotating machines. Although the above description was given for a 2-pole machine, the same concepts can be extended to describe stator winding MMF for machines with more than two poles.
Another of the desirable considerations of the present invention is the reduction in higher order harmonic magnitudes in the stator winding terminal voltage waveform. The following description provides some background information on stator winding terminal voltage waveform for concentric stator windings. For a generator operating under open-circuit stator conditions, the harmonic flux density waves produced by the rotating rotor with current flowing in it""s rotor winding (or by permanent magnets or both) would link the nested (concentric) coils of each of the stator phase windings. These harmonic flux linkages vary with time due to the rotor rotation, and therefore induce terminal voltages in the phase windings in accordance with Faraday""s Law. The peripheral spans of the nested coils as well as the number of coils influence the harmonic voltage content in the phase winding voltage waveform. Ideally, there would only be a fundamental sinusoidal component of induced voltage.
In reality, there are many higher order harmonic components of induced voltage. The higher order harmonics are not desirable in that some harmonic frequencies are known to cause humming or other extraneous noises in communication circuits located in close proximity to power lines running from the generator. Therefore, limits are placed on the harmonic content in the open-circuit terminal voltage waveform by industry standards for generators. The industry standards give xe2x80x9cweighting factorsxe2x80x9d as a function of harmonic frequency according to the degree to which the frequencies influence communication circuits. The weighting factors, in turn, are used to calculate parameters known as xe2x80x9cTelephone Influence Factorsxe2x80x9d (TIF) for which there are established maximum limits. Harmonics other than the fundamental are undesirable for rotating machine performance. Therefore, less harmonic voltage content or reduced harmonic voltage magnitudes are desirable for the design of rotating machines.
Accordingly, there is a need to reduce troublesome harmonics in the stator windings MMF waveform and the stator winding terminal voltage waveform of turbine generators.
The present invention relates to a single-layer-interspersed-concentric (SLIC) winding method. In one exemplary embodiment of the invention, the coils in each phase group of the SLIC winding may not be restricted to xe2x80x9cadjacentxe2x80x9d slots. Instead, the coils in a phase group may be located in any slots within the confines of a given pole so long as the coils are centered about a common axis (i.e. concentrically disposed around a common axis).
In another embodiment, the present invention relates to a polyphase electrical machine having one or more stators, each stator having a plurality of phase windings, each phase winding having a plurality of concentric coils. One or more concentric coils of a phase winding are interspersed with one or more concentric coils of at least another phase winding to reduce undesirable harmonics and parasitic losses in the electrical machine.
In another embodiment, the present invention relates to single phase electrical machines, wherein harmonic content and parasitic losses may be reduced by locating nested coils such that successive nested coils among a plurality of nested coils may have a span difference of greater than two stator slots.
In yet another embodiment, SLIC and concentric windings may be used for different stator winding assemblies.
In one aspect, a polyphase electrical machine, comprising at least one stator, each stator has a plurality of phase windings, each phase winding has a plurality of concentric coils. In the electrical machine, one or more concentric coils among the plurality of concentric coils of a phase winding are interspersed with one or more concentric coils of at least another phase winding to reduce harmonics and parasitic losses in the machine. Each stator of the machine includes one or more slots, and one or more stator slots comprise a conductor. Further, one or more stator slots comprises a stator bar. The concentric coils are made of insulated cable material, superconductor materials. Each coil is further made of a continuous conductor. In the electrical machine, end connection rings of the concentric coils are nested inside one another. Each concentric coil may include at least two turns. Each stator bar may be connected to the peripheral end connection rings. The two turns of the coils may be transposed to simplify joint assemblies of stator bars to the end connection rings. The electrical machine further includes means for supplying cooling liquid to cool the stator winding. Each coil of the electrical machine may include one turn.
In another aspect, a method of reducing harmonic content and parasitic losses in an electrical machine, said method comprising: providing at least one stator, each stator having a plurality of phase windings, each phase winding having a plurality of concentric coils; and peripherally interspersing one or more concentric coils of a phase winding with one or more concentric coils of at least another phase winding. The method further comprises connecting the phase windings in one of a series, parallel, or a combination of series and parallel configurations; and locating a stator bar in one or more of said stator slots. The method also comprises providing end connection rings to each concentric coil; providing each concentric coil with at least two turns; and transposing the at least two turns to simplify joint assemblies of stator bars to the end connection rings.
In yet another aspect, a polyphase electrical apparatus, comprising: at least two poles, each pole having a plurality of phase windings, each phase winding comprising a plurality of concentric coils disposed in corresponding armature core slots such that the armature core slots are formed into a nested peripheral endwinding arrangement; and at least one concentric coil of a phase winding is peripherally interspersed with at least one concentric coil of at least another phase winding to reduce harmonics and parasitic losses in the electrical apparatus.
In a further aspect, an electrical apparatus, comprising: at least one stator, said at least one stator comprising a phase winding and a plurality of slots, the phase winding comprising a plurality of nested coils, each nested coil centered on an axis of the phase winding; and wherein one or more successive nested coils among the plurality of nested coils may have a span length of greater than two stator slots to reduce harmonic content and parasitic losses in the apparatus.
In a further aspect, a method of reducing harmonic content and parasitic losses in an electrical apparatus, said method comprising: providing at least one stator, said at least one stator having a phase winding and a plurality of slots, the phase winding comprising a plurality of nested coils, each coil centered on the axis of the phase winding; and disposing each nested coil in at least one of the plurality of slots provided at least one of the plurality of slots is centered about an axis of the phase winding, and wherein one or more successive nested coils among the plurality of nested coils may have a span length of greater than two stator slots.